NiTi形状记忆合金裂纹尖端热力耦合行为研究

本文对NiTi形状记忆合金I型裂纹尖端热力耦合行为进行了数值仿真分析和实验验证。建立了包含相变和热力耦合的本构模型,通过有限元计算得到了裂纹尖端附近的纵向应变、马氏体体积分数和温度场分布,依据马氏体相变情况对裂纹尖端有效应力强度因子进行了修正,揭示了加载速率对形状记忆合金裂纹尖端有效应力强度影子的影响规律。参数研究表明,随着加载频率的增加,裂纹尖端附近温度逐渐升高,马氏体相变区域逐渐缩小,有效应力强度因子呈下降趋势,形状记忆合金表现出增韧效应,有助于减缓裂纹扩展。本研究结果对于揭示热力耦合作用下超弹性形状记忆合金疲劳裂纹扩展规律具有重要参考意义。     

裂纹与薄型夹杂物的互相干涉问题

本文研究裂纹和夹杂互相干涉的弹性力学的平面问题.一对位错和一对集中力的格林函数被分别用以形成裂纹和夹杂.所得积分方程适合于任意相对方位和尺寸的一个裂纹和一个夹杂.文中描述了裂纹尖端附近应力场的奇异性.对夹杂尖端附近应力场的奇异性给了特别的注意,并为夹杂尖端的应力强度因子作了定义.对各种不同的裂纹夹杂几何情况和不同的夹杂刚度作了数值计算.根据这些数值结果——裂尖和夹杂尖端的应力强度因子,分析、讨论了裂纹夹杂的各种几何参数以及夹杂-母体材料刚度比对裂纹-夹杂互相干涉效应的影响.     

相变增韧陶瓷Ⅲ型裂纹增韧分析

本文采用压力敏感相变准则和权函数法对相变增韧陶瓷Ⅲ型裂纹的增韧效应进行了理论预测,分别给出了静止裂纹和定常扩展裂纹相变塑性屏蔽的理论表达式。结果表明:相变体积膨胀对增韧无贡献,裂尖屏蔽来自于晶格剪切和裂纹扩展尾区的贡献。     

夹杂对两相材料界面裂纹的影响

根据界面上应力和位移的连续条件,得到了单向拉伸状态下,含有椭圆夹杂的无限大双材料组合板的复势解。进一步通过求解Hilbert问题,得到了含有夹杂和半无限界面裂纹的无限大板的应力场,并由此给出了裂尖的应力强度因子K。计算了夹杂的形状、夹杂的位置、夹杂的材料选取以及上、下半平面材料与夹杂材料的不同组合对裂尖应力强度的影响。计算结果表明夹杂到裂尖的距离和夹杂材料的性质对K影响较大,对于不同材料组合,该影响有较大差异。夹杂距裂尖较近时,会对K产生明显屏蔽作用,随着夹杂远离裂尖,对K的影响也逐渐减小。另外,软夹杂对K有屏蔽作用,硬夹杂对K有反屏蔽作用,而夹杂形状对K几乎没有影响。     

改进的Irwin模型及裂尖塑性区对断裂行为的影响

本文研究了小范围屈服条件下I型裂纹尖端塑性区对断裂行为的影响.Irwin模型假设塑性区外奇异应力场分布是弹性解的平移,并将塑性区的一部分加上原有裂纹视为等效裂纹.这样得到的等效应力强度因子总是大于相应的线弹性解的应力强度因子,这与塑性区的增韧作用相悖.为了考察塑性区对裂纹尖端附近应力分布的影响,本文提出在塑性影响区内,裂纹延长线上奇异应力分布与线弹性奇异应力场静力等效的原则.在此基础上建立了改进的Irwin模型,并导出了衡量塑性区屏蔽效应的显式表达式,定量地解释了塑性区的屏蔽效应,本文结果与基于相变增韧理论的方法得到的结果在趋势上一致.     

陶瓷材料相变增韧尺寸效应的力学分析

利用计及相剪切效应的陶瓷材料本构关系和裂尖场的双参数表达式对平面应变Ⅰ型裂纹定常扩展过程进行渐近分布。基于渐近场和能量积分，得到韧性增值的估算公式，并且根据数值结果和实验现象进一步讨论相变剪切塑性和应力非奇异顶的影响。     

相变增韧陶瓷Ⅱ型裂纹增韧分析

本文采用压力敏感和权函数法，对相变增韧陶瓷Ⅱ型裂纹的增韧效应进行了理论预测。分别给出了静止裂纹和定常扩展裂纹相变性屏蔽的理论表达式，结果表明；相变对静止裂纹无增韧效应，纹尖端屏蔽来自于裂纹扩展尾区的贡献。     

可压缩粘弹性材料Ⅱ型动态扩展裂纹尖端场

为了研究粘性效应作用下的动态扩展裂纹尖端渐近场,建立了可压缩粘弹性材料II型动态扩展裂纹的力学模型,推导了可压缩材料Ⅱ型动态扩展裂纹的本构方程.在稳态蠕变阶段,弹性变形和粘性变形同时在裂纹尖端场中占主导地位,应力和应变具有相同的奇异量级r-1/(n-1).通过渐近分析求得了裂纹尖端应力、应变和位移分离变量形式的渐近解,并采用打靶法求得了裂纹尖端应力、应变和位移的数值结果,给出了应力、应变和位移随各种参数的变化曲线.数值计算表明,弹性变形部分的可压缩性对Ⅱ型裂尖应力场影响甚微,而对应变场和位移场影响较大.裂尖场主要受材料的蠕变指数n和马赫数M的控制.当泊松比ν =0.5时,可以退化为不可压缩粘弹性材料Ⅱ型动态扩展裂纹.     

采用显微网格法研究韧性材料平面应力状态下裂尖附近的应变场

用显微网格数字图象处理方法，测量了韧性材料平面应力条件下Ｉ型单边裂纹尖端附近的应变场。实验结果表明：在裂尖两侧存在与裂纹方向夹角约为５０°的大变形带。最大应变位于离裂尖一定距离的大变形带上，随着载荷增加，最大应变的位置逐渐靠近裂纹尖端。应变峰值轨迹线在裂纹顶端呈放射状，载荷增加时，应变峰值的轨迹线位置并不改变。     

三点弯曲试样动态应力强度因子计算研究

利用Hopkinson压杆对三点弯曲试样进行冲击加载,采集了垂直裂纹面距裂尖2mm和与裂纹面成60°距裂尖5mm处的应变信号。根据裂尖附近测试的应变信号计算试样的动态应力强度因子,并与有限元计算结果进行比较,结果表明由于裂尖有一段疲劳裂纹区,通过裂尖附近应变信号来计算动态应力强度因子时,如果裂尖位置确定不准及粘贴应变片位置不够准确对计算结果将带来很大影响。因此利用应变片法计算动态应力强度因子时,为了获得更准确的计算结果,在实验后应对试件裂纹面进行分析测量,重新确定裂尖位置,必要时需对应变片至裂尖距离进行修正后再计算动态应力强度因子值。     

压电圆柱形夹杂对邻近裂纹三维应力场的影响

研究了压电复合材料薄板中压电圆柱形夹杂与邻近宏观钝裂纹间的相互作用.重点分析了外加电场、裂尖与压电圆柱形夹杂间韧带长度对裂尖三维应力场的影响.计算结果表明:在不同的外加电场作用下,压电体不仅能改变裂尖张开应力的大小,还能改变其分布.所得结果对进一步探讨线弹性介质中裂纹的启裂控制有参考价值.     

各向异性复合材料中界面屈服/脱粘对裂纹扩展的影响

本研究针对层状复合材料中正交于层合面的裂纹,研究裂纹前方层状界面发生屈服或脱粘现象对该裂纹前沿应力场的扰动。通过利用叠加原理,借用滑移型位错密度表征界面的屈服或脱粘。利用Chebyshev数值积分法求解相应的位错密度的奇异积分方程,得到沿界面屈服/脱粘区域的位错密度分布及裂端区应力场。结果表明,若层状复合材料界面为发生屈服或脱粘,将减弱独立层裂尖的应力奇异性,进而抑制独立层中裂纹的扩展。     

弹性T项对裂尖参数和裂纹扩展路径稳定性的作用

研究了弹性T项在主裂纹与近裂尖微空洞干涉问题中对裂尖参数和裂纹扩展路径稳定性的影响作用.利用“伪力”法,并通过解决主裂纹与近裂尖微空洞干涉问题,对远场纯1型载荷和远场弹性T项载荷下,该干涉问题中弹性T项的影响作用进行了分析从数值结果可以看出：由于空洞的存在;释放了弹性T项所引起的应力,弹性T项对裂尖参数;应力强度因子和J积分都有直接显著的影响,因而,它对该载荷下的裂纹扩展路径的稳定性有控制作用。     

Role of elasticity in elastic–plastic fracture: Analytical bi-linear crack-tip fields and finite element analysis

A solution for Model-I plane strain crack tip fields in a bi-linear elastic–plastic material is presented. The elastic–plastic Poisson's ratio is introduced to characterize the influence of elastic deformation on the near tip constraint. Attention is focused on the distribution of elastic/plastic strain energy in the sensitive region of the forward sector ahead of a crack tip. The present study shows that the elastic strain energy can be higher than the plastic strain energy in this sensitive sector while large amount of the plastic strain energy develops outside this sector around the crack tip. The effect of elastic deformation in this sensitive region on the structure of crack-tip fields is considerable and the assumption in some important solutions for crack-tip fields reported in literature that the elastic deformation is small and can be ignored is therefore not physically reasonable. Besides, finite element analysis is carried out to validate the analytical solution and good agreement between them is found. It is seen that the present solution with T-stress can properly describe the crack-tip fields under various constraints for different specimens and an analytical relation is established between the critical value of J-integral, J_c, and T-stress for elastic–plastic fracture.     

On the driving force for crack growth during thermal actuation of shape memory alloys

The effect of thermomechanically induced phase transformation on the driving force for crack growth in polycrystalline shape memory alloys is analyzed in an infinite center-cracked plate subjected to a thermal actuation cycle under mechanical load in plain strain. Finite element calculations are carried out to determine the mechanical fields near the static crack and the crack-tip energy release rate using the virtual crack closure technique. A substantial increase of the energy release rate – an order of magnitude for some material systems – is observed during the thermal cycle due to the stress redistribution induced by large scale phase transformation. Thus, phase transformation occurring due to thermal variations under mechanical load may result in crack growth if the crack-tip energy release rate reaches a material specific critical value.     

Field structure at mode III dynamically propagating crack tip in elastic-viscoplastic materials

An elastic-viscoplastic mechanics model is used to investigate asymptotically the mode Ⅲ dynamically propagating crack tip field in elastic-viscoplastic materials. The stress and strain fields at the crack tip possess the same power-law singularity under a linear-hardening condition. The singularity exponent is uniquely determined by the viscosity coefficient of the material. Numerical results indicate that the motion parameter of the crack propagating speed has little effect on the zone structure at the crack tip. The hardening coefficient dominates the structure of the crack-tip field. However, the secondary plastic zone has little influence on the field. The viscosity of the material dominates the strength of stress and strain fields at the crack tip while it does have certain influence on the crack-tip field structure. The dynamic crack-tip field degenerates into the relevant quasi-static solution when the crack moving speed is zero. The corresponding perfectly-plastic solution is recovered from the linear-hardening solution when the hardening coefficient becomes zero.     

Effect of crack-tip shape on the near-tip field in glassy polymer

The physical nature of a crack tip is not absolutely sharp but blunt with finite curvature. In this paper, the effects of crack-tip shape on the stress and deformation fields ahead of blunted cracks in glassy polymers are numerically investigated under Mode I loading and small scale yielding conditions. An elastic–viscoplastic constitutive model accounting for the strain softening upon yield and then the subsequently strain hardening is adopted and two typical glassy polymers, one with strain hardening and the other with strain softening–rehardening are considered in analysis. It is shown that the profile of crack tip has obvious effect on the near-tip plastic field. The size of near-tip plastic zone reduces with the increase of curvature radius of crack tip, while the plastic strain rate and the stresses near crack tip enhance obviously for two typical polymers. Also, the plastic energy dissipation behavior near cracks with different curvatures is discussed for both materials.     

Field structure at mode Ⅲ dynamically propagating crack tip in elastic-viscoplastic materials

An elastic-viscoplastic mechanics model is used to investigate asymptotically the mode Ⅲ dynamically propagating crack tip field in elastic-viscoplastic materials. The stress and strain fields at the crack tip possess the same power-law singularity under a linear-hardening condition. The singularity exponent is uniquely determined by the viscosity coefficient of the material. Numerical results indicate that the motion parameter of the crack propagating speed has little effect on the zone structure at the crack tip. The hardening coefficient dominates the structure of the crack-tip field. However, the secondary plastic zone has little influence on the field. The viscosity of the material dominates the strength of stress and strain fields at the crack tip while it does have certain influence on the crack-tip field structure. The dynamic crack-tip field degenerates into the relevant quasi-static solution when the crack moving speed is zero. The corresponding perfectly-plastic solution is recovered from the linear-hardening solution when the hardening coefficient becomes zero.     

Crack growth resistance of shape memory alloys by means of a cohesive zone model

Crack growth resistance of shape memory alloys (SMAs) is dominated by the transformation zone in the vicinity of the crack tip. In this study, the transformation toughening behavior of a slowly propagating crack in an SMA under plane strain conditions and mode I deformation is numerically investigated. A small-scale transformation zone is assumed. A cohesive zone model is implemented to simulate crack growth within a finite element scheme. Resistance curves are obtained for a range of parameters that specify the cohesive traction-separation constitutive law. It is found that the choice of the cohesive strength t₀ has a great influence on the toughening behavior of the material. Moreover, the reversibility of the transformation can significantly reduce the toughening of the alloy. The shape of the initial transformation zone, as well as that of a growing crack is determined. The effect of the Young's moduli ratio of the martensite and austenite phases is examined.     

温度变化对热压电介质裂纹尖端力电损伤的影响分析

由连续介质损伤力学的基本理论出发,引入力电损伤变量并建立了一个热压电介质断裂的损伤本构模型.再由虚功原理导出了求解这类含损伤的关于热力电耦合问题的有限元方程.通过数值计算,分析了温度改变对裂纹尖端力电损伤的影响规律.